The present invention relates to a transmission device and a transmission method for transmitting signals by accommodating time-division multiplexed signals in signals in a packet format.
Backbone networks of communication carriers are being progressively switched and shifted to full-IP•Ethernet networks in recent years. In such circumstances, already-existing backbone networks employing the time-division multiplexing technology (based on SDH/SONET, for example) and newly emerging backbone networks transmitting information in a packet format (based on IP•Ethernet, for example) are coexisting.
In order to eliminate inefficiency of equipment, maintenance, etc. due to the above coexistence of different types of networks, a lot of studies are being conducted in order to consolidate the backbone networks based on SDH/SONET into those based on IP•Ethernet by packetizing the conventional SDH/SONET signals, PDH/DS-n signals, etc. into IP•Ethernet packets. As a result of the studies, the T-MPLS (MultiProtocol Label Switching) technology has been established and specified in ITU-T Y. 1370.1, Y. 1371, Y. 1381, etc.
In low-speed paths (TU-11, TU-12, etc.) specified in ITU-T G.707 (standard of SDH), however, the number of bytes of one path in one STM-0 frame (51.84 Mbit/sec) is 27 bytes (25 bytes of genuine user traffic area excluding management bytes) (TU-11), 36 bytes (35 bytes of genuine user traffic area excluding a management byte) (TU-12), etc. Thus, depending on the length of the unit of the packetization, the occupancy ratio of non-payload areas (overhead added for packetization, inter-frame gaps, etc.) in the whole communications traffic increases and that of the payload decreases. As a method for preventing the decrease in the occupancy ratio of the payload, it is possible to temporarily accumulate the low-speed paths (basically in units of frames) for a sufficiently long time and then packetize the accumulated paths into one packet as specified in Section 9.1.1 of ITU-T Y.1413 (03/04). For example, by accumulating TU-11 paths for ten STM-0 frames and packetizing the accumulated paths, the occupancy ratio of the payload can be multiplied by ten. The aforementioned ITU-T Y.1413 recommends accumulation of N units (N: positive integer, 1 unit=27 bytes (1 frame)) and packetization of the accumulated N units into one packet (see ITU-T Y.1413, ITU-T G.703, ITU-T G.707, Telcordia GR-253-CORE, etc. for details related technologies).